Yes, you can add a command object to your report and put any SQL you like in it. Then the report uses the resulting data to print your report. This is found on the Data tab of the Database Expert. Click on the Add Command node under your current connection.

If you want to find expert tricks for optimizing your report for SQL and writing custom SQL statements, check out chapter 10 and 11 in my book Crystal Reports Encyclopedia. You'll be surprised at all the great stuff I packed into two chapters.