How can i use main-menu items as a fields in selectbox in drupal 7
First, create a view that references the nodes you want - e.g. that have a parent item with node id x (or whatever the criteria is for those nodes). Get that view right, and the rest is easy.

Upload and enable either the references, or the entity reference module http://drupal.org/project/entityreference or http://drupal.org/project/references

In your content type, choose a node reference item and select widget. You can base the option list on your view.