While in Jhāna to walk or (reaching) Jhāna is not possible while walking?

Based on Piya Tan's analysis of the sutta Venāga,pura Sutta (A 3.63) and his quote of the traditional commentaries, we can see that it's possible to sit, walk, stand or even recline while remaining in jhana attainment, up to the fourth jhana.

For full translation of the sutta by Piya Tan, please see Venāga,pura Sutta (A 3.63). For alternative translations by Ven. Sujato and Ven. Bodhi, please see SuttaCentral's AN 3.63.

While in the (Sangha) Uposatha Sutta (A 4.190), the Buddha describes the mental state of the saints in general, “Bhikshus, there are monks in this community of monks who dwell having attained the state of devas... the state of brahmas... the imperturbable... the state of noble ones,” in the Venāga,pura Sutta, the Buddha describes his own mental state, but omitting mention of the formless attainments (ānejja-p,-patta).

The Buddha says that he attains these states “whenever I like, with neither difficulty nor trouble.” The Commentary explains that “(attains) with neither difficulty,” that is, “(attains)without difficulty” (akicchā,lābhi), means that it is attained “without unsatisfactoriness” (adukkha,lābhī), that is, painlessly. The phrase “nor trouble,” that is, “without trouble,” means “obtaining abundantly” (vipula,lābhī) (AA 2:293). These words come from a popular stock passage on the saint’s ease at attaining dhyana, thus:

Here and now, attaining to the four dhyanas, the higher mind, as he wishes, without any difficulty, without any trouble [in abundance], he dwells happily.

The Buddha adds that when he is in each of those states (evam,bhuta), whether he is sitting, walking, standing or reclining, he is similarly in a state that is heavenly (dibba), or perfect (brahma), or noble (ariya). In connection with the first two states — the heavenly and the perfect — the Commentary says:

Having attained the four dhyanas, he does the walking, that is, the “heavenly walk.” Having arisen from the attainment, too, he does the walk. Standing and the rest, too, are like that. (AA 2:294)

In this exegesis, supramundane dhyana (lok’uttara jhāna) is meant, that is, the dhyana of the Buddha and the arhats, who can conduct their daily lives, even sleep, in dhyana. Moggallāna, for example, could at once get into the fourth dhyana to perform various psychic wonders.

Venāga,pura Sutta (A 3.63)

Acting postures base on five strings, color sound smell taste and body-feeling (temperature hardness movement) because we can't do any physical acting without five senses, so acting postures are the opposite of Jhana directly according to MN13 MahadukkhakkhandhaSutta.

You can try it, taping your eyes and stuffing your ears then run. And you can understand it by memorizing the 4th chapter of AbhidhammatthaSangaha, mind processing chapter.

However, it is possible to walk with Jhana by the genius people who practice each Jhana again and again until getting Jhana Mastery level (vasi).

How they can do it?

They switch the mind process with unimaginable speed.

Attaining Jhana a few millisecond then thinking of five strings a few millisecond to walk then attaining Jhana... switching like this until the end of walking.

This is why MN22 putting Posture section after Anapanassati section, and DN10 Subhasutta putting Posture sub-section after IndriyaSamvara sub-section in AdhiCittaSikkha main-section.

Also, KN Kuddakapatha MettaSutta...

And let him too with love for all the world

Maintain unbounded consciousness in being

Above, below, and all round in between,

Untroubled, with no enemy or foe.

And while he stands or walks or while he sits

Or while he lies down, free from drowsiness,

Let him resolve upon this mindfulness:

This is Skillful Practitioner here, they say.