A signature is a non-reversible (but verifiable) cryptographic transformation on some input data. Signatures are not copyrightable since
- they do not require any creativity to produce (their creation is completely mechanical) and they do not contain any creativity themselves (it's just an arbitrary numerical value), and
- any copyrightable elements (indeed, any identifiable properties at all) from the input are completely and deliberately absent in the output of a signature algorithm (this is in contrast to compilation, which is purely mechanical but preserves the copyrightable, creative elements of its input)
Thus, anything a copyright license has to say about what the copyright holder does and does not allow you to do does not pertain to the creation of a cryptographic signature. In copyright terms, its analogous to the creation of a cryptographic hash. (And in case you're wondering if copyright can disallow you from distributing a hash: no, it cannot; you are always allowed to do that under copyright.)
As for whether or not you are allow to attach a signature to a binary (as opposed to keeping it separate), that's perhaps a slightly different legal question. That might or might not create a derivative work (even though the signature itself is devoid of copyright, some jurisdictions might view such an alteration as a derivative), but even if it is, any open source or free software license will allow you to make and distribute such an alteration.
In case you are concerned that your production of a signature might misrepresent you as the author of the work, that's not what signatures are for. Signatures just say, "Entity X had access to this work and asserts its contents are Y." After a recipient verifies the cryptographic validity of the signature and sees that the contents of the file are indeed still Y, the recipient can be sure it has not been changed from how it looked when Entity X had it, even if it first passed through the hands of Entities Y and Z.
No one ever asserted that Entity X was the author of the file. We can only be sure that Entity X had access to the file and chose to make a cryptographically unforgeable assertion about the state of the file at that time.