Here are the signature guidelines,
The maximum number of lines in a signature is 5.
No external links or email addresses will be posted in signatures.
Do not put any kind of advertisements in signatures.
Signature text should comply with our regular posting rules.
Signature text that is too long or obtrusive will be removed.
This would include overly creative use of punctuation, color, etc. which make the signature too flashy
If your signature is in violation of any of these new rules, the Staff may remove it without prior notice.
So I would say yes as long as no links are included.
We do have a section in you user profile to include instant messaging names etc. although twitter is not there...