I can only suggest you a workaround approach!
Why don't you try to create the desired table(foreign in this case) directly using a SQL Developer? But two things here..
1. You have to create your own sequence to generate id column.(Not mandatory, only if you are planning to have one)
2. Since you mentioned it is only a temporary temple, you wont be bothering about cache mechanism. Just in case, you are concerned about cache, you can give an entry to SystemInfo table mentioning it as a foreign table.
This isn't the most elegant solution, but it avoids interacting directly with the DB. I've created a table with more than 15 columns like this:
1) Create table with max number of columns (15) using Admin Tab > Management Tools > Sites Database > Add Table
2) Export the table using Catalog Mover
3) Edit the .html file that gets exported and add as many new columns as desired. That's done in the first <tr> section in the file.
4) Delete the table from WebCenter Sites
5) Import the updated .html file using Catalog Mover. Now you have your own object table with more than 15 columns.
You can also create the tables programmatically using the CatalogManager APIs - have a look at ftcmd=createtable. There is a GSF facade over creating tables too which is really easy to work with:
and here's an example of how to use it (check out the install() method):