Perhaps you can explain more what you want to do, of course the best place for excel files is in excel, perhaps even online excel, what do you mean by in tiddlywiki?. Excel can be imported and turned into Tiddlers, tags and fields, but excel also imports/exports CSV and tiddlywiki can import/export csv. There are other ways to generate output from excel that can be used in tiddlywiki, eg screen capture, html files, now with Evans formula and other tools you can also do many things in tiddlywiki you can do in a spreadsheet.
Regards
Tony