From the Lojban web site FAQ (http://www.lojban.org/files/brochures/faq.htm):
lujvo: compound word
- selbri :- brivla | tanru
- tanru :- brivla brivla+
- brivla :- lujvo | gismu | fu'ivla | cmavo
- lujvo :- tanru
A lujvo is composed of a sequence of rafsi, concatenated according to a rather large set of morphological rules. Two strings of characters that are both a lujvo are considered the same word if and only if their rafsi belong to the same set of words in the right order. This means that mitpavycinglepre, mitpavycinletpre, mi'urpavycinglepre and mi'urpavycinletpre have no difference in meaning, and may alternate freely. However, it is only common to list the first alternative in dictionaries, because of the four, that's the one selected by the Tansky-Lechevalier scoring algorithm.
Of course. Anything we can't express with gismu and combinations thereof, we borrow as fu'ivla.
If it can't be expressed as a fu'ivla from ANY language, it cannot be (or has not been) said by humans.