No. You either draft the hub in a word process or directly into capsules.
If you compose on the fly directly into capsules, there are still other important items off to the right that need to be addressed: group, tags, etc. With this method, you really don't have more than a general impression of what the hub is about until you're done. When you've finished, you can tease the reader cogently and succinctly in 250 characters or less, and use a couple of keyword phrases in the process.
If you compose in a word processor, then you have the summary to start with and can enter it first, but only if you remembered to write it. I write the article, then the introductory first paragraph, and finally the summary teaser.
Extra credit pet peeve:
>>The first paragraph is an *introduction* to prepare the reader for the body of the hub.
>>In the publishing industry, a *summary* is the concluding paragraph(s) to succinctly review the hub contents.
>>In the publishing industry, a *teaser* (HP summary) is used to pique the reader's interest, whet his appetite and hook them into wanting to click and read.