Am I allowed to submit to a journal, a paper that was accepted to a conference but not presented yet?
If I understand your question correctly, you're planning to submit the journal paper after receiving the decision from the conference. In that case, if the journal is ok with considering an extended version of a conference paper, the conference organizers are probably not in a position to say anything against that. However, you have to follow the standard referencing rules when finalizing the journal paper:


The material that already appeared in the conference is not "new" for the journal paper, but has been published previously in the conference paper. Accordingly, the relevant section in the journal paper should be formulated as a review of existing results.
You must cite the conference paper from that section.