I want to extrude away some threads on hardware models that I imported as STEP files.
When I go to do this the "remove material" option is grayed out.
Your imported geometry is surfaces only, no solids. With no solids in the model, Creo has nothing to cut. If you switch the feature to a surface, you will be able to cut the surfaces.
Solidify the imported surfaces and you will then be able to cut away the threads. This may be as simple as redefining the import to be a solid, or may require repairing the import.
Is STEP file imported as solid or surface? Surface model is one of the common reason for such issue.
If it is a surface model, try solidify that prior extrude.
A few things...
Your method does work. Can you try to extrude all the way up to the hex head surface? You can extrude to a reference. Add material instead of trying to remove it.
I've attached a video of another really cool option. It's the one I teach to users most of the time. Based on what you select, the remove command works differently. If you select all of the surfaces that make up the thread, they will be removed more quickly than what I showed in the video.
As others have said, most likely your imported model is surface geometry. Generally Creo imports as solids if he can, which means your import probably has some errors. This leaves you with two options:
If you need help solidifying your geometry, there are a few techniques you can use. The first one I normally try is the repair tool in Data Doctor. Here's a quick video I made for somebody else last week on the topic. If this doesn't work, I can suggest some other options.