Entering a value into a grid cell: I know that it is possible to read grid cell values thanks to the new command GetGridValues. But is it possible to input values in grid cells?

A command similar to GetGridValues ist not available for grid cell input. But input into grid cells is possible via SAP GUI scripting. You can use ApplyGUIScript or  - with GuiXT Controls - CallVB / CallVBS. Activation of SAP GUI Scripting in SAP profile and in SAP GUI is necessary for this solution.