Yes, all maps from the original Stronghold can be played in Stronghold: Definitive Edition.

All you need to do is copy your maps from the original maps folder to the new Stronghold: Definitive Edition maps folder location:

1. Browse to the original Stronghold maps folder.  Default location is \Documents\Stronghold\Maps.
2. Select and copy all the maps you wish to transfer.
3. Browse to the new Stronghold: Definitive Editions maps folder.  Default location is C:\Users\{username}\AppData\LocalLow\Firefly Studios\Stronghold 1 Definitive Edition\Maps.  Alterntively click on the "Open Settings Folder" within the in-game "Game Options" to open the folder.
4. Paste the maps into the new folder.