An "Insert signature" button is already present in the latest SVN builds, and will be in the next general release, I'd guess. It will insert the signature for the identity you are composing with.
To use multiple signatures, I'd guess you will need to build a separate identity for each signature. When composing you would need to select the identity first, which will determine the signature to use. You could do this even now with the current release version, the selected sig would be auto-inserted (no "Add" button).