Numbering is....not perfect.
You can do this just fine. Select your bullet points, choose Format > Numbering and Bullets, click the numbering style tab, select a number format, click OK, and you're good.
But what if you want a normal unnumbered line in the main text area, then start with a numbered list that, of course starts a one? You can't (not easily) because numbering starts on the first item in the main text area. This is what happens if you select the second and subsequent items and apply numbering.
There is one hack and one workaround.
The Hack: Offset the first line so it starts a zero.
1. Select the items you want numbered. Choose Format > Bullets and Numbering, Njumbering Type tab, select your numbering format as usual.
2. Then click the Customize tab. Select Level 1 in the level list at the left.
3. Type 0 (zero) in the Start At field.
4. Click OK. Now the unnumbered item thinks it's 0.
Can you set it farther back like -1? No.
The Workaround: Just type your numbers.
It's easier than struggling to make Impress do something it won't, and of course there are no formatting snafus if you go back and forth between Impress and Powerpoint.
The Other Workaround: Does it really need to be a numbered list?
If not, use bullets.