In this paper we describe semi-automatical extending of the Czech WordNet lexical database (48,000 literals in 28,000 synsets) by translation of English literals from existing synsets in Princeton WordNet. We make use of a machine-readable bilingual dictionary to extract English-Czech translation pairs, search the English literals in Princeton WordNet and in case of a high-confidence match we transfer the literal into Czech WordNet. Along with literals, new synsets parallel to the English ones and identified by ILI are introduced into CzechWordNet, including information on their ILR (Internal Language Relations) such as hypernymy/hyponymy. The paper describes the parsing of the dictionary data, extraction of translation pairs and the criteria used for estimating the confidence level of a match. Results of the work are 36,228 added literals and 12,403 created synsets. An overview of previous similar attempts for other languages is also included.