Sorry for the issue with pasting the text, but It is not possible to change that pop-up. It was purposely placed there to strip out the back-end formatting. Previous versions of SiteBuilder did not have this feature, but then the formatting that was hidden in Word docs cause all sorts of other issues.
If you have a small amount of text, it is probably just as easy to type it directly into the SiteBuilder text box. If you have very large amounts and you want to keep the font, size, style, color, layout etc. of everything that you have in the Word doc, you can try to save the Word doc as a web page. Then open the web page in your browser, right click (you'll see the html script for the page), select all, and copy the the html code. Then insert an html box into the web page and paste in the code. This method works well if you have lots of text or need to embed a table form Word or Excel.
If you download the original site builder and use it just for what you want to cut and paste, it works fine. I wanted to rearrange a list of product descriptions and links and couldn't do it in Plus with losing font color and links. When I went to the old Sitebuilder, it copied and pasted just fine--keeping font style, color, and link. Try that if you have to copy and paste. It cut my work from a possible 6 hours to about 1.