Can we add a inside an tag?

Yes you can.

HTML4 has this to say:

<!ENTITY % heading "H1|H2|H3|H4|H5|H6">
<!--
  There are six levels of headings from H1 (the most important)
  to H6 (the least important).
-->

<!ELEMENT (%heading;)  - - (%inline;)* -- heading -->

And %inline; is:

<!ENTITY % inline "#PCDATA | %fontstyle; | %phrase; | %special; | %formctrl;">

And %special; includes <span>.

The current HTML has this to say:

Content contents
Phrasing content

And Phrasing content includes <span>.