Anvil Studio, like virtually every other MIDI sequencer I'm aware of, will allow you to enter MIDI Continuous Controller messages, other wise known as CC#xx.
To get to any of the voices on the DGX-650 the first thing you need to do is to go to "download.yamaha.com" and choose your country.
In the little menu that comes up enter "dgx650" or use the prompt and search.
You'll get a list of manuals that you can download in pdf format. Choose the "Data List".
Once downloaded, I suggest you save it.
From page five onwards you will see an instrument list.
This includes first the Program (Voice) Number as named by Yamaha.
Second the two Bank Select messages called MSB and LSB. They are in fact CC#0 and CC#32 respectively.
Then the MIDI Program Number. In the manual it's numbered from 1 to 128, but in fact you may have to subtract 1 before entering it in Anvil, i.e. 0 to 127.
It's inserting these CC#0 and CC#32 with the appropriate value, in the MIDI file before the Program Change (using the that will select the voice you require.
So, select (highlight) the track/channel you want to edit.
Go to "View/Event List". Check where the Program Change message is. It may need to be moved later in the file.
Look at its absolute time, in brackets. Double click on the event and change the "Time of Event" if necessary.
Select from the top menu "Edit" then "Insert MIDI Event".
Put the correct MIDI timing in to occur before the Program Change.
"Kind of Event" becomes "0:BankSel", channel will remain the same, and for a normal melody voice the value will be "0".
Insert another MIDI event, place it just after the previous one, but this time select "32:BankSel LSB", and enter the LSB number from the Data List,
e.g. for "Natural Grand Piano" enter "122". Then make sure the Program change is correct and do the same for any other channel to set the correct voice.
That should be it.
It looks much worse written out than it actually is to implement.