I am not a lawyer, and this is not legal advice. I'm also not particularly familiar with MURS, and generalizing from other information.
According to Wikipedia, MURS (as many other US unlicensed radio services do) requires the use of type accepted equipment.
(Terminology note: radio services are CB, FRS, GMRS, MURS, amateur, commercial, etc.)
This means that the only radios you can use on MURS are those which the FCC approves for use on MURS. So, no, your modded radio cannot legally be used. (Modified equipment is only useful on the amateur bands or otherwise on frequencies which you have been granted a specific a license to use.)
And in these services which require type-accepted equipment, FCC generally only approves equipment which cannot be used for licensed services — the widely understood rationale being that they don't want to trust the users of the type-accepted equipment not to use the other functions.
Thus, there is no radio which you can legally use in the US which allows transmitting on both amateur frequencies and unlicensed service frequencies.