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[Scatty]

Most if not all old pre-Windows era games for Ms-Dos were never meant nor suited for modifications by players, open modding became an accepted standard much later, around 2004 or so.
There might be some fan-made tools for a few Ms-Dos games on fan sites if you search on Google, but that is more of an exception that regularity. Not meaning to dissuade you if you'd like to mod a game, but it might be not an easy thing to do.

[Smiling Spectre]

Definitely not all, Scatty. There always was WADs for Doom, scenarios in wargames, level editors for ball-breaker and Lode Runner games, lakes for BassTour, etc, etc. Some games even tried to be "game editors", more or less successfully.

But yes, _most_ of games are not so easy to modify.

[Neville]

My advice would be to look for the manuals of said expansions. Some were easy to use, but others (like the Test Drive 2 addon disks) were a nightmare without their manuals.