This is not unusual. I think you are suggesting adding a plate under the bearing block to bolt to, which is in turn bolted to the extrusion rails. This is fine, but will raise your ballscrew by the thickness of the plate - is that a problem?

If it is a problem, these blocks normally have both vertical holes and horizontal holes for mounting. So instead you can use the horizontal holes to bolt into a thick plate/block which is next to the bearing block, and is in turn then bolted vertically down onto the extrusion. The trade-off this time is a small loss of ballscrew travel equal to the thickness of the block/plate.

If none of these options work, you can drill and tap straight into the face of the extrusion (i.e not using the rails) - but you will need to check if there is enough material to tap into and you avoid the slots.