Hello Denise Latreille
I was actually just playing with this example to understand why this happens and have an example here
It seems that you can add 'null' ONLY if part of this is capitalised- AnNULLable or even AnNullable
The only way it appears to possible to add the term you want exactly is to export your Termbase to Excel (using the Glossary Converter) add the new Term and then convert back to SDLTB using the Glossary Converter....
Then the term is available as you need it to be.
The problem appears to be that 'null' is a command in MultiTerm- which means it is simply ignored by MT ( null=zero) and looks like to be a bug.