Nope, you would need to make two separate versions.
If i learn DX11, can i make the game compatible with DX9? I'm guessing you can but it's worth asking. I mean i'm sure its close enough syntax wise so i wont be super lost but.
Personally, assuming you plan to release the game in a few next years, I would ditch DX11 and go for DX9. DX9 is good enough and all games are written for this version anyway (sometimes with optional DX11, enchanted, version), so your games won't look inferior.
That is plain wrong. DX11 has feature sets that let you target DX9, DX10, DX10.1 or DX11 hardware. So yes, you can use DX11 to target DX9 class hardware... of course that still won't work on WinXP.
It makes no sense to use DX9 anymore, DX11 is a far superior and more elegant API. Many new games coming out these months are already DX11 only, I'll be surprised to see a DX9 game coming out in 4-6 months.
XP is not relevant anymore (just have a look at the steam hardware survey) and if the trend is confirmed it is going to disappear in few months.. unless you are producing budged games for very old hardware, but in that case, there are much better options than using a native DX9 engine.