How to add fraction of a month to a date in Excel?

Would this work for you?
The original date is in cell A3, the returned date is on row 4.

=LET(
    StartDate, $A$3,
    EndPoint, B2,
    EndMonth, EDATE(StartDate, EndPoint),
    MonthFraction, EndPoint - INT(EndPoint),
    DaysToAdd, INT(DAY(EOMONTH(EndMonth, 0)) * MonthFraction),
    Result, SUM(EndMonth, DaysToAdd),
    Result
)