When building your template, if you would like to map to specific Constituent Records as the Solicitor/Canvasser, you can use the Constituent ID as a means to do so.

*Since First Name and Last Name/Surname are generally required, you should fill them in (with either a Source Field or a Default Value), but set both rows to not update for either New or Existing Constituents.  

*Then, in the Constituent ID (or ID) field, choose a Source Field to map to (I have Last Name in the view below):

*Select the Field Settings (in the far right column of the row for Constituent ID). Create a Data transformation mapping the your field to the Constituent ID of the record that you would like to be the Solicitor/Canvasser).  Shown below is a sample based upon the Last Name/Surname.

This transformation will link the Solicitor record directly to the existing record in The Raiser's Edge.