Kato does indeed make an inspection pit.
You get two of them as part of the 23-228 Track Side Accessories kit.
http://www.1999.co.jp/eng/10003182Kit comes with 2 inspection pits, 2 wide wood rail crossing and 2 skinny wood rail crossings, some sign boards, ladders, gates, etc.
It requires you to cut the roadbed off the Unitrack, so it may just be easier to transfer to the Peco inspection pit and then back to Unitrack. You would just need to get the Peco track length cut to match.. which is probably easier than cutting the roadbed off Unitrack.