Yes. It is possible.
Create a two-row table.
Apply your style to a cell in the first (header) row of the table, but not in the second row.
Save your table as a quick table. This is a type of building block.

To insert a new one, insert it from quick tables. If you assign a unique name to your quick table you can use that name and F3 to insert the table rather than going through the menus in Word 2013 and later. (I usually start my building block names with an underscore _ so they show up at the top of menus.)
Where can Building Blocks be stored?

The behavior of Word in tables is when a new row is added, it uses the formatting of the row above it.

If normally the lower rows use numbering at a lower level of the multilevel list, you could apply that style to your second row.

I noted that you said your numbers should not change. This is not how automatic numbering works in Word. If something numbered is added or deleted before your entry, the numbering will change. That is part of the meaning of "automatic."