I have been changing some email addresses to hyperlinks (mailtos) in a spreadsheet.

To do so I made the hyperlink bar visible. Then the following procedure _sometimes_ works.

1. Click on a cell containing a mail address. (The address appears in the editing bar; contents of the 
hyperlink bar at this point variable).

2. Triple-click the mail address in the editing bar to select it all. (At this point, if all goes well, that address 
appears in the hyperlink bar in the text box, and the URL box goes blank.)

3. Alt-press and drag the selected text from the editing bar to the URL box. (Now it's in three boxes: two 
on the hyperlink bar and one on the editing bar)

3. Press (don't click) the link icon on the hyperlink bar and choose 'as text'. (At this point a mailto link 
appears)

This procedure doesn't always work! 

Problem a: just double-click a single character in the editing bar at stage 2. That single character appears 
in the text box of the hyperlink bar, and no amount of clicking or selecting will make the text box change.

Problem b: if the URL box doesn't go blank at stage 2 -- it may, for example, retain an earlier URL from an 
earlier attempt -- it's really hard to make it go blank. One way that usually works is to click in another 
spreadsheet cell and start again from step 1.

Problem c: can't paste into the text box of the hyperlink bar. Pasted text goes elsewhere, usually into the 
editing bar.