I have some objects and I want converter in only object and will be the background. How do I do it?

You can group objects together (Ctrl+G once they are selected),
move them to bottom (End)
create a new layer at the bottom (Shift+Ctrl+L for the layers panel)
move group to the bottom layer (Shift+PgDown)
and lock the bottom layer on the layers panel.
Then you can work on the layers above without risking any unwanted edit of the background.

Well now THAT was some amazing mind-reading!!

Of course I can be wrong, and I'm also thinking original poster might want to change the page borders too (Shift+Ctrl+D if he is still reading this)
and/or the problem is related to raster images where you cannot add transparency effectively to make other objects a part in its background...
who knows?

I actually have no idea what roirosal wants. Because of "converter" I think maybe something about Trace Bitmap or Object to Path. But really, no clue for me.