Importing (at least in MS Project 2000) is done through File, Open. Select Excel as the type of file, and then your Excel file that has your resources & Hours.
Then you'll need a Map - if you didn't add any columns in the excel spreadsheet, you should be able to use the same map you used for the export to excel. Otherwise create a new map.
From your description, I can't tell if the resources were exported along with the tasks or if you added them while working in Excel. This could be cause problems trying to go back into Excel, if you've spelled a resource name wrong, etc.
Where it can als get messy is if you added rows to your Excel spreadsheet (if a task has multiple resources, etc.)
Need I say BACK UP EVERYTHING before attempting this?
IMHO, I would advise against this type of thing in the future, if all you were doing is added work effort hours to resource assignments. I'd suggest using a view such as the Task Usage view to see your tasks, resources assigned and then add your Work effort in there. I personally don't think it is worth the import/export hassle.
Hope this helps...
JanK, PMP