I don't think there's a way within Houdini, but you can do something like this with a macro app. Autohotkey on Windows, for example, lets you remap any mouse button to a keyboard button.
If you are on Windows, install Autohotkey and use this script:
This will remap the mouse middle button to the grave accent button (only within Houdini). You can change that to any key or combination of keys you wish.
#IfWinActive, ahk_exe houdini.exe
`::MButton