if i buy an ipad without cellular can it be added later?

No you cannot.

You can not physically add wifi to the iPad but there are various mifi devices you can use.

Also, if you have a suitable iPhone you could use it as a Personal Hotspot, as long as your carrier supports that feature.

What are you asking? Are you asking if you can buy an iPad 3G without a cellular plan then add the cell plan later? Or are you asking if you can buy a wifi-only iPad then add cell to it later?