Unfortunately, FTM does not handle historical place names as well as some other applications do. Others may have ideas for better ways to do this but here's how I handle them:
1. In Places view, find the current location by selecting it from the list on the left side.
2. Make certain FTM indicates the current location is resolved. If not, resolve it.
3. Over on the right, copy the GPS/Latitude Longitude data which is in the Location field.
4. Back on the left side, select the historical location.
5. On the right side, paste the GPS/Latitude Longitude data (of the current location) into the Location field of the historical location.
6. Back to the left side, right click on the historical location and select "Ignore Place Warning".
FTM will then indicate the historical location is resolved and it will show up on the map in the same spot as the current location.
I wish FTM at least had the ability to add a note to a location, or somehow tag it with from/to dates, so that there would be a way to clearly indicate the dates of a historical location's existence.