Additional Reviewers after Document Already Signed

After one group of reviewers has signed a document, can I add additional reviewers/signatures?

If not, can I add the attachment to a portfolio and an additional page with reviewer signatures as a separate document in the portfolio? Can a portfolio have digital signatures attached to it as well (i.e. send out the entire portfolio for digital signatures)?

I am trying to have a first group of reviewers approve some invoices, and subsequently do a packet with a check request to Accounting--the whole thing (reviewer-signed invoices, check request) needs to be in one packet. I'm not sure if Portfolio is the best way to handle this, or if there's another quick trick I'm overlooking?

Thank you in advance!