Perhaps. And perhaps not. Many would elongate or voice ("voice" in the linguistic sense meaning activate the vocal cords) the m in ham - not just the lengthen the a - in a way that they wouldn't when saying hamster, in which they might pronounce the "m" as a glottal stop (the reason why hamster is often misspelled as "hampster") or as a voiceless bilabial consonant.

Regardless, it's a red herring, because vowel length can also refer to "long" vs "short" vowels as in Bake vs. Back. Thats a different, and in my view more common, meaning of vowel length

Elongating the voice is exactly what I mean by an allophonic difference.

As for 'hamster', no one (that I know of) pronounces the /m/ as a glottal stop, although people sometimes epenthesize a voiceless consonant [p] (by devoicing the end of the /m/). Where some English dialects get a glottal stop is for an intervocalic or word-final /t/ (in addition, of course, to the glottal stop in the middle of 'oh-oh' and 'uh-uh'). I've also heard American English speakers glottalize word-final /k/ and sometimes /t/, but a glottalized stop is not the same as a glottal stop.