I'm trying hard not to question why you would want such an inconsistent and illogical numbering structure.
But what you might want look into is a style separator. To make use of a style separator, you have to
- Insert two paragraphs (e.g. first paragraph = heading text, second paragraph = other text)
- Click at the end of the first paragraph
- Hit CTRL + ALT + ENTER
This will insert a style separator at the end of the first paragraph and shift the second paragraph to the cursor position. You can not click into the heading (the part before the style separator) and apply a heading style to it and then you can click into the part after the style separator and apply and type of different style to it.
Note however, that if you try to apply a list numbering to the second part (after the style separator), Word won't insert an additional number. So, for this first line you'd have to insert the number manually.