I am not sure if FBReader on the 360 has the same config screens as on other Linux or Nokia OS2008 versions, but to adjust the spacing between paragraphs you want to go to the FORMAT tab/screen and set the options for the REGULAR PARAGRAPH. You should see a number of settings. Change the values for SPACE BEFORE and SPACE AFTER. On my Nokia N800 I set those both to "5" (remember they work together. To work with those settings I also set the LINE SPACING value on the same tab/screen to 1.2 and have all my indents to "0".
FBReader uses STYLE based config and it's super once you get over the hump of having so much config control. In fact the STYLES tab/screen works with the FORMAT settings for each "paragraph style" within a book format. And the settings are universal not on a per book basis.
Not having the 360 yet, I can't help you find the correct settings screen/tab and the only versions of FBReader are the one which runs on the Linux like OS2008 on the Nokia N800 as also the pretty pitiful Windows XP version. But my guess is all work the same way even if the access to the settings are different, meaning what you need to change should use the same names even if you access the settings differently.
Hope that helps.