Filemaker Pro can easily do this. You would use merge fields in text string.
"Dear " & <<salutation>> & " " &LastName
The above is an example how it would start. Try it and post back with more questions.
That won't quite work.
You can compose the letter as layout text and insert the fields as merge fields into the block of layout text:
Dear <<YourTable::Salutation>> <<YourTable::LastName>>
You can set up a calculation field that returns text with an expression such as:
"Dear " & Salutation & " " & LastName
You can also set up "template" records where a large text field contains the text, plus placeholders for each designated field:
Dear <Salutation> <LastName>
And then a calculation can replace the placdholders with data from your fields:
Substitute ( TextFIeld ; ["<Salutation>" ; Salutation ] ; ["<LastName>" ; LastName ] )