I've been wondering, how does the Imperial Army get their troops, during the Skyrim period especially? Is service voluntary or is there conscription?

I have never heard of conscription in the series, only Recruiting. I am sure it is Volunteer based.

This would probably best be put in the lore forums - they'd be more knowledgeable on this subject than we would.
I'd assume that it's all voluntary though. I've never seen or heard anything to suggest otherwise - besides, most of the Imperial faithful seem to be proud of their service.

Etymologically speaking, I'd say that at least one point in their history they conscripted people, as the word Legion ultimately derives from the Latin word for 'levy'.

Naturally, this doesn't necessarily mean anything for Tamriel.

They're all volunteers during the Skyrim era. And IIRC from Morrowind they get the benefits (piece of land in a conquered territory) that real Roman soldiers did.