While no doubt Pfeil will respond with a far more sophisticated response....
You could accomplish this in any number of ways.
The most simply way is probably Just to do the following:
Create a command.
under the command name you will see a "when I press" radio box. Check that and you can select the trigger key
Under the command, create a while loop (Other -> Advanced -> Add a loop start -> Single Condition While loop.
Within the following dialoge select Device State on the far right.
Choose your device and button.
Select while pressed.
Within the loop created, add either a "quick input" or "keypress" command. You can set the timing "between keystrokes" in the respective dialogues
Now when you press the command trigger, your command which launch, which so long as you continue pressing the key, it will loop with the timings specified.
Start Loop While : Keyboard Key 'A' Is Pressed
Press T key and hold for 0.1 seconds and release
End Loop
The above command, when "A" is assigned as the launch trigger will effectively make your "A" key function as your "T" key. for example, and if "A" is held, it will repeat at the desired rate.