Not that easy to answer, it would help to let us know which condenser you have.
You'll have to build some kind of support to fit under/inside the condenser where you could put some stops.
But you won't be able to realize good darkfield for higher magnifications that way anyways.
Maybe this could give you kind of an idea:https://www.ebay.com/itm/264323855175
You could also try to find an affordable phase contrast condenser,
you would get nice and easy dark field on the lower magnifications with the ph stops.
And you could upgrade your BH to 1-2 ph objectives later..