If the keyboard does not talk directly with Bold, then You surely need something between - of course You can build some DIY project 
But i think, it may be simpler just implement some connecting logic in Homey ( hub/pro/…).
About keypads - seems, not so many of them walking on Market Street and waiting to take to home. Quick Google giving thread here and on top of search is frient’s kaypad NB! Not tested, just googled and noticed, that here is also Homey app with possibility to start flow with entered pin.
