I'm not exactly sure what symbol you're looking for, so I'll use the û for now. I'm also using LO 3.4.4, so hopefully these instructions will work with your version of LO:
In a new LO Writer document, select Insert -> Object -> Formula. Type in your formula (mine looks like this):
a + <where the symbol will go>
To add the û, select Tools -> Catalog. This (confusingly) brings up a window named 'Symbols'. In that window, select the 'Special' symbol set, and click the 'Edit' button. There's a whole lot going on in this window, perhaps too much, but it is possible to navigate it.
Under the 'Font' label, choose 'Bitstream Charter' (or another font that contains the glyph you need), and then scroll up and down in the character selection area until you find the Lowercase u-umlaut character (û). Click on it to select.
To add the character, click the 'Add' button. Make sure not to click either of the 'Delete' or 'Modify' buttons, or you might change/remove some of the existing symbols, and we don't want that to happen!
Back in the 'Symbols' window, you can select the û character, and then click the 'Insert' button to insert the symbol into your formula:
a + û
Do you want to add a hat to any letter, or only the specific letter U: Û?
Well, in fact I also want to do that with the letter v, V and i I because they are frequently used in electronics literature.
Well, there is a Unicode character for Û, but not for V. You can use a combining character: V̂, but I think you'd be much better off just using an inline equation that can actually support the correct notation.