You can, yeah. If you try to do this through the editor (select planet, press escape --> editor --> edit planet), it won't work because for a tidally locked world the engine is implicitly ignoring the rotation period.
Instead you'll need to tell SE to use the rotation period you give it by exporting and editing the planet file directly. The edits you need to make are extremely simple, but the exporting process is a bit more involved and I'll refer you to the MOD: Creating a Planet thread for those details. But basically all you have to do is add in a line for "RotationPeriod" just above the lines for Obliquity and EqAscendNode, and give it the value you want. Refer to the code of a non-locked moon for comparison, or see below:
Color (1.000 1.000 1.000)
Note this is purely for procedural objects, not solar system bodies.
Hope that helps, but feel free to ask if you run into any problems or have more questions. And welcome to the forum.