You need to use advanced flex mode and edit your flex controller DMX element.

https://developer.valvesoftware.com/wiki/Blender_Source_Tools_Help#Advanced_mode

and then just export it like normal?

how do i even edit flex controller element?

You can use Blender's built-in text editor. The Tools will generate a Text when you click on the button mentioned in the documentation.

so i set my controller source to dmx file with flexes, generate and then edit text file, and then export?

Yes.