Yes, it is possible. This is an investigative technique performed by undercover Police officers under fake identity that infiltrate, establish and maintain contact with the target group of people. According to Section 158e of the Code of Criminal Procedure, this measure may be employed only in case of investigating certain serious crimes: a) crimes, for which the Criminal Code prescribes a sentence of imprisonment with the upper limit of 8 or more years, b) crimes committed for the benefit of an organized group, b) other specific crimes, namely: machinations in insolvency proceedings (Section 226 of Criminal Code), breach of regulations on rules of economic competition (Section 248 (1) e), (2) to (4) of CC), arranging advantage in commission of public contract, public tender and public auction (Section 256 of CC), machinations in commission of public contract and Public tender (Section 257 of CC), machinations in public auction (Section 258 of CC), abuse of power of public official (Section 329 of CC), accepting bribes (Section 331 of CC), bribery (Section 332 of CC), indirect corruption (Section 333 of CC) c) crimes, prosecution of which is stipulated in international treaties binding the Czech Republic.
As concerns "infiltration by an informer of the requested State": No, it is not possible in this sense (in respect of international judicial cooperation). The Police Act no. 273/2008 Coll. defines (in Section 73) an informer as a person providing information to the Police in such a way that this cooperation with Police authorities remains undisclosed to third parties. The informer may receive financial reward for his services and he may be provided with cover identification documents. However, any information provided by an informer may not be used as evidence in criminal proceedings (although it may lead the Police to the initiatation of criminal proceedings). However, an alternative would be to use a Police officer as an agent for the required infiltration task within the framework of covert investigation.