Yes, using by inserting the following Microsoft Word merge field in the mail merge document:
{ IF { MERGEFIELD SitePostalAddress } <> "" "{ MERGEFIELD SitePostalAddress }" "{ MERGEFIELD SiteStreetAdress }" }
Open the Microsoft Word mail merge document from the PRS Document Manager.
Since you can't just enter a Word field directly as it's displayed, use the Insert Word Field button on the Mail Merge toolbar to insert the If...Then...Else... item and the Insert Merge Field button on the Mail Merge toolbar to insert SitePostalAddress and SiteStreetAddress fields into the Then and Else clauses respectively.
Take a look at the Seminar Invitation document in the PRS Demo (Tools->Document Manager menu command), the envelope address uses this example.