There's no way to import Word or other templates… but, imho, there's no need for this.
Avery 8859 cards are 3.5" x 2" cards on a US letter sheet. There're 10 cards by sheet.
So, from BCC, create a new document then from the Inspector>Document tab choose "Change Layout…" (or select File>Change Layout… from the menu bar).
From there, select Avery, US letter and Category: Businees Cards then choose Avery 10 Clean Edge Business Cards (3-1/2 * 2in) cards model (yes, this is not exactly the same than 8859, margins seem to be different, in particular there's an inner gutter):
Click on the Create Custom… button:
Now, "all" you have to do is adjusting correctly the horizontal and vertical margins/spacings:
Caution: 0.5" Horizontal Spacing I used here, is just an example, you have to measure margins on the sheet itself to get the right margins sizes then adjust left margin and horizontal spacing.
At the end don't forget to rename this custom template to something like Avery 8859.
To test this, create a card with a colored border (use a rectangular smart shape with a black border/stroke/line of 1.00 pt but no fill). Print this on a "normal" paper sheet (to avoid wasting glossy paper) then place this printed sheet over a 8859 glossy paper sheet and see if borders coincide with cards boundaries.
Hope this helps.