I'm not sure about appending "-CM", but I'll bet someone else here will know. From a technical standpoint I suppose you could use a Calculated field and put in +"-CM", but I am not sure if that will actually create a Credit Memo in the eyes of Sage. I have reversed signs before, by using a Temp Field to bring in the amount and then use a Calculated Field to multiply the Temp Field by -1.

AP Invoice Data Entry doesn't appear to have any fields that indicate the type of invoice, i.e. standard, credit memo, debit memo, etc.  So you are free to place what you want (up to 20 characters) in the InvoiceNo field.  In order for VI to handle multiple rows in your source file correctly, you should point a temp field using the Replace operation to your column containing the invoice number.  It should be set to assign on key assigns only and should be set to reset.  Then set your InvoiceNo field to use the Calculated operation and append "-CM" to the temp field.

As for reversing the sign, rclowe's suggestion is what generally works.  Just make sure to set up the temp field pointing to the amount as a line level temp field that is numeric and have it reset.  Then use it in the calculation on the DistributionAmt field.