One of the things you can do is turn an assembly into a joined part.

If you did so, perhaps COSMOSxpress would treat it as a single item and let you analyze the weldment.

Upgrading is a good idea, however. Regular COSMOS has many features that COSMOSxpress lacks. You can have more than one type of restraint, or load, and there are many other aspects that make it much more useful.

But for right now, try the "join" idea. You can find it in Solidworks online help.

Regards.