If you read the tcllib page, you will find information about content requirements, submission guidelines, etc. for your software.
However, recently tclguy summarised the process on news:comp.lang.tcl as:
See also tcllib for the policies and discussions relating to another module of this project, which has tcl-only procedures.
List here the suggestions for inclusion in tklib
Remember to list only items that come in Tk scripting - compiled widgets like tkimg, tkhtml, tktable, tktreectrl, blt, tix, vuw widgets, tkdnd, etc. belong in the various Batteries Included distributions.
Another systematic source for tklib-appropriate ideas are ports of the add-ons and widgets that Perl/Tk and Tkinter (and perhaps PMW) build in beyond the core ones provided in Tk itself.
I encourage people to list other useful Tk related widgets and add-ons that are not already a part of some other extension.
In particular, look around this wiki - I suspect there's a number of wonderful possibilities here.
For instance, check out Commonly requested widgets - if you can find well written versions of these, putting them into tklib would be a wonderful idea. Do be careful to verify the licensing status of the code, and GET PERMISSION FROM THE AUTHOR before adding code to the library though.
AM Things I can add: