there is actually some way to put your screenpack in 1.0. it not as easy as drag-and-drop but still you'll have your screenpack safe.
1. download mugen 1.0
2. copy all of your Chars, Stages, Fonts, Sounds in the corresponding folders of 1.0.
3. go to this new mugen's Data folder. create a new folder and name it the way you want. lets call it YourFolder.
4. Go to your old mugen's data folder, copy all the files and paste them into YourFolder.
5. Then, in this new mugen's data folder open the mugen.cfg file. replace the current "motif = ..." line with
motif = data/YourFolder/system.def
6. Now for the hardest part. Go to your old mugen's Mugen.cfg and copy the following sections:
Otions, Rules, Config, Debug, Arcade and Input.
Paste these sections into your new mugen's mugen.cfg by replacing the existing ones.
7. now it should work.
if you forgot some fonts, chars, or whatever, Mugen Error report will tell you what is missing.
P.S.: I'm sure the official documentations has its words about installing Motifs, so you might check Mugen 1.0 docs for Motifs. Motifs are all to save your old mugen screenpacks in 1.0