Need 3 signatures but signer #3 will be assigned at a later date

I am filling out an application to be a mentee and it requires 3 signatures - mine, my bosses' and, at some point in the future, if my application is accepted/approved, my mentor.  Since the application has to be approved (by an internal group) before a mentor is assigned, is there a way to make a signature field for the mentor on the document?  

Or is there a way my boss and I can sign, then I can make a flat file or something like that, that the mentor, once assigned, will be able to sign the document. 

I've attached a blank copy of the application so you can see what I'm talking about.

Thank you! Carol