Merging separate accounts not so easy, however select your single easiest account, then IMPORT email from each of your other accounts into your selected main account.
Is more setting up needed IF you wish to email out from main account using SENT BY your chosen other account address.
BTW an not an MS user, and the service decides whether the above can be done.
Telstra currently accepts other USER addresses for accounts, etc...
Did this when needed change from bigpond.com ages ago as available storage space for emails failed to keep up with needs.
Found Yahoo and Google easily imported and stored bigpond and other emails, with room to spare without need to log into bigpond.com.
Side effect of this was encouragement to keep using bigpond email addresses :-)
Since changed to #linux #openSUSE with #GNOME a decade ago, not enthusiastic at suggestion return to MS.