How can I change ProjectSuite.Variables when using testexecute?

You can't do it using TestExecute. But you can edit project files (.pjs or .mds), which contain all project suite's and project variables. This files have XML structure and it is easy to parse all required data. Only problem - string variable values are stored in HEX format. So you will need to encode/decode it to/from unicode string. Integers/doubles are stored as is.

[!!!] You should be VERY CAREFUL with your project file data and fully understand what are you doing on your own risk. [!!!]