Can I use a Prism data table as a spreadsheet, and write a formula into a cell?

No. A Prism table is not a spreadsheet and no calculations are possible within a data table. But Prism does let you do some data manipulations, and can interact with Excel in surprising ways...

Prism analyses can manipulate data

Prism's Analyze button does more than initiate statistical analyses and curve fits. Use it also to transform and normalize data, and to do computations to subtract baselines or combine columns. In all cases, the results appear in a new results table. With Prism 5, this table has green grid lines, telling you that this results table can be used as a data table -- you can  graph the results or analyze them further.

Prism Windows lets you embed or link Excel files

Prism Windows (but not Mac) lets you link or embed Excel files within a data table (version 3 and later). Set up Excel to do the calculations you want, copy the results, and paste onto a Prism data table.

When you paste/embed, a copy of the entire spreadsheet becomes part of the Prism project. Double-click on the pasted data on the Prism data table to open the spreadsheet for editing. Change data on the spreadsheet, and corrections are made automatically on the Prism data table. When you paste/link, double-clicking will open the linked Excel file. 

More about linking and embedding.