Cconference papers are of different types. Most are either original research or Review/overview/updates etc. Original research is usually "aired" in a conferences either as posters or oral presentations as a step towards publishing it in a peer-reviewed journal. Other conference presentations are based on review of the literature with or without personal contributions to the subject. These can be made into more comprehensive or if they address a specific aspect, a new development, or address special populations (region, subgroups, etc). They too can be published but better write to the editor in chief before your full submission. Best of luck.
Normally the content published in journals has a deeper purpose in different investigative submissions at conferences. Look detail the aspects not covered by the work submitted in the conference and look to add existing discussions on the issue through articles published in the journal you want to submit.
It is depending on the kind of the conference, some of them publish the paper in a special issue in some journals, in this case you will not able to publish it again. The other only give a certificate of attending the conference, in this case you can publish it.