WordMail does not include an Insert | Signature command like the regular Outlook editor. If you want to be able to insert a signature into a WordMail message manually, you can create an AutoText entry for each signature. You can then insert them with the Insert | AutoText command.

Or, create multiple signatures the normal way (Tools | Options | Mail Format) and make one the default. If you want to use a different signature for the current message, right-click the signature that Outlook inserted automatically. From the pop-up menu, select either the name of the signature you want to use or E-mail Signature to create a new one. I personally prefer this technique. This is also the technique you need to use if you want Outlook 2003 to automatically change your signature when you switch accounts on an unsent message.

Are you using Word as the editor?  If yes, then you need to define one
signature as the default.  Then, once you have composed your mail, you can
right click on the signature and show the list of your defined signatures to
select from.