Having a packet
cPacket *pk
how can I obtain the bit representation of it? For example, in the form of
std::bitset<pk->getBitLength()> pk_bits;
My final goal is to apply an encoding scheme to the packet, i.e. Reed-Solomon encoding.
As @rcgldr commented, a simple cPacket in itself does not hold any data, at least not in the sense of how real packets do. And it's not necessary in most models, because they operate on a higher, more abstract level, which makes working with them easier, and running them faster.
The information that travels between the nodes of the simulation is what you put in the fields of your messages (preferably custom made using the message compiler of OMNeT++, from .msg files).
This is, however, completely independent of the bitLength/byteLength properties of the cPacket class, which is just a number that can be set to any value for any message.
You can, of course, choose to model a realistic protocol by adding fields to your message that correspond to a real(istic) network protocol header, like TCP or IP, or even something you just made up. But this still doesn't provide any (reliable) byte-sequence-like access to the contents, because it is not always trivial how the individual fields should be serialized into simple octets.
To achieve this, INET for example has separate *[De]Serializer classes for a number of its custom message types. You can do the same with yours if you want.
A simpler solution would be to represent any payload in the packet by adding an std::vector<unsigned char> or even an std::bitset if you prefer that. And just treat that part separately from the easily accessible fields, applying any encoding on its content.
And finally, just like with any question like "how to add an encryption library to a simulation and use it to transform packets": Are you sure that adding a real byte-by-byte encoder/serializer/etc. to a simulation is the right choice to achieve what you're trying to do? I mean, it could be, and it's possible, but there might be better/simpler/faster ways. In terms of modeling.