Yes, contractions may be used, as explained in Braille Formats §21.6.2: "When entry words are shown with syllable breaks, insert the word in contracted form with no syllable breaks at the margin. Follow this (after one blank cell) with the word showing syllable breaks and using contracted braille." There is also a print/braille example: Sample 21-3 Glossary with Diacritics on page 21-30. However, Braille Formats §20.4. Diacritic Pronunciation explains: "20.4d. Do not use contractions when a modifier applies to one of the letters within a contraction."
Braille Formats 20.4.1–20.4.4 along with Samples 20-6 –20-14 provides specific details for diacritic transcriptions. Also, in the Rules of UEB, Section 4.2, you'll find the complete list of modifiers, and examples for their use.—eng and schwa pronunciation symbols are explained in UEB §4.4.