Unfortunately there is not a great way to do that using the current backing framework. You could try to work around the limitation by creating a fake asset in your prices timetable that represents incoming cash, and then have that asset double in value at each rebalance time step. You could then treat the growth of the fake asset as the cash contribution, pull it out, and allocate it with the rest of your portfolio.
However, this workflow is not recommended and it would mean your summary metrics (e.g. total return, Sharpe ratio, etc) would all be incorrect.