Is it possible to get Seti Classic units to appear in my sig?

SETI classic is not a BOINC project so no.

Since that's static info you could add it as text beneath the image. And you should set up your signature using the Account preferences link from "My BAM menu". Then you don't have to type the image link in each post.

Gruß,
Gundolf

You could try out the mundayweb-stats if you want to show them.
This would look like my one here then:

If I remember right they ported over the SETI Classic workunits over to our BOINC Credits back some time ago (years). I remember being contacted about that with regards to my account.

They are showing as a memento to past times in your account at Seti. They were never converted to Credits, as it was impossible to do, given the variation in run-time per WU, even back then. So the amount of work units you've done, plus the amount of time they've taken is showing in Your Account.