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

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

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