Can ArcGIS Pro use Python toolboxes written for ArcMap?

This from ESRI:

Your existing toolboxes, models, and scripts can be reused in ArcGIS Pro. However, if you use arcpy.mapping, you may need to make adjustments to work with project files (.aprx) and maps, instead of map documents (.mxd). In addition, ArcGIS Pro uses Python 3, so it is possible that you may need to make some changes to your scripts.

There are still functions not available in ArcGIS Pro so if your script uses them then I would guess you are out of luck until they are added. For instance, Data Driven pages is not available.