I guess it depends on what you’re trying to add and whether or not you truly mean “entries” or just individual items. Files/folders, URLs, and strings are pretty straightforward. For just about anything else, you’d need to write a plug-in.
If it’s a file or folder, you can select it in Quicksilver and use the “Add to Catalog” action.
There are also multiple ways to pre-create an entry that you can add individual items to on-the-fly whenever you want.
One way is to add a “File & Folder Scanner” entry that points to a text file. For the “Include” setting, pick “Text Lines”. You can add URLs, filesystem paths, or arbitrary lines of text to this file however you want, and they’ll appear in the catalog (after the next scan).
If you just want to add files/folders, you can install the OpenMeta plug-in and add an “OpenMeta Tags” entry for a particular tag and then at any later point, assign that tag to a file to have it added to the catalog. I went into more detail about that at
http://projects.skurfer.com/posts/2013/tagging_tips/ under “One-off Catalog Entries”.