You could use a table driven pattern, which allows you to generate a pattern using X-Y coords. What I did as a test was to set up a quick Excel spreadsheet to generate the variable pitch pattern. Then copy the numbers generated by the spreadsheet and paste them into a new notepad document.
In notepad change each line to this format:
1mm 0mm
1.1mm 0mm
1.21mm 0mm
....etc
The 1st column is the X dimension and the 2nd is the Y dim.
Save this notepad file as yourFilename.sldptab.
In SW (hang in there, this is almost over) insert a coord sytem on your base slot, oriented so that the X axis is the direction you want your pattern to go. Insert->Pattern/mirror->Table driven pattern..., browse to the sldptab file, pick the coord syst you just created, and pick the base slot for Features to copy. Click OK to generate the pattern.
Hope this helps... there may be an easier way, but this is the 1st that popped into my head.