There are several freeware, shareware, and commercial utilities that allow you to regenerate the DDS for physical and logical files and, depending on the utility, printer and display files.

Alex Nubla's Retrieve Database Source (RTVDBSRC) Utility
This free command re-creates a DDS source member from an existing physical or logical file.
http://www.geocities.com/~alex_nubla/rtvdbsrc.htm

Projex400
This shareware toolkit includes the RETRIEVE DDS SOURCE (RTVDDSSRC) utility.
http://www.projex.com/projex400/products.html QUSRTOOL Retrieve Physical File Source (RTVPFSRC) Utility
QUSRTOOL was free in V3R1 and previous releases of OS/400; for later versions of OS/400, it's now sold as TAA Productivity Tools from http://www.taatool.com.

SSTG DDS Decompiler
This commercial utility can re-create source for compiled DDS physical, logical, printer, and display files.
http://www.sstglobal.com/dds.htm

Ancient iSeries NEWS Utilities
The November 1990 issue had an article "Get to the Source of It!" that included the RTVDDSSRC command that retrieves source for database files. This article is 13 years old, and neither the source code nor the article text is online. I've made the source code available at http://www.iseriesnetwork.com/noderesources/code/clubtechcode/RTVDDSSRC.zip. Please note that I make no guarantees about this code.

The April 1992 issue had the article "Re-create Printer Files with Ease." The article is not available online, but you can obtain the code at http://www.iseriesnetwork.com/code/index.cfm?fuseaction=ShowAllIssueCode&IssueSelect=Past&IssueYear=1992&IssueMonth=04. Again, old article and old code.