Okay the answer is in two parts. UG is interactive with excel using spreadsheets so for that you need to look at the documentation for Spreadsheets. If you can do so then this may be the simplest option and that won't involve extra licenses for products you may not have purchased.
Yes you can launch external applications via grip, there is a command for it. However passing data back and forward from grip to excel is going to be the tough part. Grip will not read an excel formatted file. It will output a formatted table that excel could open as a text file. You may want probably create a program using VB or similar to automate that process, and lauch that instead, (this is what I did in the past). If you want to interact in the opposite direction with UG you'd need to save you results from excel as a text file and have grip open that to read in the results.
Hopefully somebody will post here with a solution for UG open programming, which how uses VB anyhow and might be a heck of a lot simpler.
Try the spreadsheets thing first. Let us know what you're trying to achieve and what version of UG you're using.
Regards
Hudson