Federal stipends are certainly helpful in supporting research and experience for early investigators. Even so, there are strict limits on amounts and sometimes a PI wants to reward extra work with extra pay. This is allowable, but the supplement cannot come from another federal source. Consider NIH Grants Policy section 126.96.36.199 which states:
Kirschstein-NRSA fellows receive stipends to defray living expenses. Stipends may be supplemented by an institution from non-Federal funds provided this supplementation is without any additional obligation for the fellow. An institution can determine the amount of stipend supplementation, if any, it will provide according to its own formally established policies governing stipend support. These policies must be consistently applied to all individuals in a similar status regardless of the source of funds. Federal funds may not be used for stipend supplementation unless specifically authorized under the terms of the program from which funds are derived. Under no circumstances may PHS funds be used for supplementation.
An individual may use Federal educational loan funds or VA benefits when permitted by those programs as described in Other Income: Educational Loans or GI Bill in this chapter.
If your department chooses to supplement the stipend of a fellow, be sure that your source of supplementation is not another federal award. Further, be sure there is no federal flow-through (not a subcontract with federal origins, for instance). Feel free to drop us a line if you’re not sure!