Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
addition [əˈdɪʃən] n (MATH) сложе́ние; (thing added) добавле́ние; (to collection) пополне́ние
in addition вдоба́вок
in addition to в дополне́ние к +dat
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.