Referencing questions of a repeatable question group from another question group is possible but with some limitation. if you reference a numeric type question, you will get the sum of the question across all rows, if you reference a text, you will get a concatenation.

For numeric type you can limit the summation using the calculated answer type and filtering out irrelevant row. 
In any case, this is regardless of being an Excel group. Repeatable questions groups behave the same regardless of the source of the information.