Back Door TSP Contributions

I was curious if there were any possibilities to pay into my TSP account without directly being withdrawn from my monthly check?

For context, I have been in the Army for 6 years (4 enlisted and 2 as an officer). I have been contributing 5% of each paycheck into my TSP since enlisting. I also have been putting max contributions into my Roth IRA and add a little money to an emergency fund every month.

I will be attending medical school on the HPSP scholarship this August and have been informed by my recruiter that, even though I am being paid through MyPay, I will no longer be able to contribute to my TSP for the duration of medical school due to my pay being a stipend and not salary.

I would like to continue to contribute to my TSP for these 4 years and was curious if there were any “back door” methods to continue these contributions. Also, if it is possible, would I still receive a match for my deposits?

I am not sure what the restrictions and/or actual rules regarding this are. There was not much to go off of online and the guidance I got from TSP, chain of command, and recruiter have all varied drastically. Any advice would be greatly appreciated. Thank you!