How to decrease indent in openoffice impress?

If you place the cursor on or just to the right of the bullet and press shift-tab, you will decrease the indent.

I don't know how to automatically extend text to another slide, but if you use the outline menu, you can see the text on several slides and can copy/paste or select/drag lines from one slide to another. You can also use the promote icon on that menu to decrease the indent.

Inside the slide, choose a paragraph (or some of them; just put the cursor in a part of its text or select some paragraphs) with a marker or numbering and:

• alt+shift+right arrow ident (ident to right)
• alt+shift+left arrow unident (ident to left)

I don't know how to do what you want in the second question, but I recommend you to reduce font size on the text box to fit the slide.