To access UG from excel the way I have seen it done was to create a hyperlink within excel. Assuming that you're running windows, because you're running excel, then so long as the system is set up to launch NX as the defualt for .prt files then creating a hyperlink to the file will cause it to open in NX when you click on the hyperlink.
In testing my system preferred that I start NX first.
The application I saw this used for was created as part expert system cum PLM tool for a company that I once worked for a few years back. Because they had a system that assigned a project number per product and derived sub part numbers and tooling item numbers by sequential increments based on the root number their system simply parsed that to a known file and directory path which information furnished a hyperlink. It was simple but effective.
Depends what you are trying to do. I normally would try and create a csv file from Ug then import that in. You can spawn a UG session using a run_journal.exe command but you need to write a Ug journal file to do what you need to do in UG. This could be launched in a script file or in a macro in excel.
The run journal executable is in nxroot/ugii/run_journal.exe