First, you perhaps need to understand that “.dat” files are often application-specific data - that is not necessarily intended to be human readable; these files may require the App that created them to open or import data from them.
Assuming that the files that you wish to import are actually comma-delimited data (i.e., a “.csv” file), changing the file extension may make the file readable by other Apps and programs.
With release of iOS/iPadOS16.x, the native iOS/iPadOS Files App now allows you to see the full filename and associated file-extension - and freely alter the filename and extension information. The tags appended to your post suggest that you have an iPad Air3 - this model being capable of update to iPadOS16.
Once updated to iPadOS 16.2 (this being the most recent release), you’ll be able to manipulate the filename extension; if your “.dat” files are actually “.csv” files, you’ll be able to easily resolve your immediate difficulty, allowing your spreadsheet data to be imported by the Numbers App.