Writing a journal paper based on work already published in conference paper
From the SAGE link you provide:

Conference abstracts, posters and presentations
Subject to the journal's policy, manuscripts based on papers that have been presented at conferences may be considered for publication as long as they have not been published and provided that you still retain the rights to the manuscript. The journal editor may review whether the version of your article considered for publication is materially different from the work you presented at a conference and/or whether publication in the journal will enable your article to reach an audience that the conference paper did not previously reach. Prior publication of an abstract or poster presented at a conference will generally not impact the manuscript's eligibility for publication.

A similar policy is enforced by Elsevier (at least AIJ) and IEEE. Prior publication at a conference does not disqualify the content for publication in the journal, but the journal version usually needs to be more detailed or add new information. The standards of a journal are normally higher than those of a conference in terms of completeness of the exposition.
So in general it is fine to extend a conference paper into a journal paper, but you should always check the particular journal's policy, in case it has a different clause that overrides the publisher's policy.
Sometimes, the submission process of the journal specifically asks whether the paper is an extension of a conference paper. Even if it does not, you should make the editor aware of this fact (for example in the cover letter), and cite the conference paper in the journal paper, explaining what the latter adds on top of the former.