Hi Alexiei;
PowerBuilder can do this natively using its InkEdit Control and or InkEdit DataWindow. However you would have to be running the PB application on a tablet or laptop that has a pen input device.
PowerBuilder can also do this on any iOS or Android device if your PB application is deployed through Appeon. Appeon supports have writing directly to the InkEdit DW or IE Control.
Once either of the above alternative capture a signature, the standard old UpdateBlob command will do the trick in saving the signature to ant DBMS table that supports Blob data types.
The final alternative I can think of is to use a 3rd party control and/or OXC/DLL that interfaces your PB Application with a signature device.
HTH
Regards ... Chris