Haven't brewed a Dubbel yet, but I was under the impression you wanted fermentation to be over with before moving to secondary, so not sure of the purpose of adding additional yeast, unless Dubbel's have a knack for killing off yeast somehow. Secondary fermentation I think is a bit of a misnomer in itself, it's just a secondary vessel to aid in clearing and flavor additions, as far as I knew. I can't imagine it will do anything bad to it, but it sounds like you'd be wasting a smack pack.
Bottled: All Hallow's Eve Pumpkin Ale
Kegs: Sunset Orange Kolsch Redux
Future: Black IPA Redux, possibly others
2013 Previous: Sunrise Honey Rye Pale Ale, Dark Skies Black IPA, Sunset Orange Kolsch, All Hallow's Eve Pumpkin Ale, Maple Vanilla Brown Ale, Raining Hops and Dogs IPA, Starry Night Vanilla Bourbon Porter
Dark Skies Black IPA - 2nd Place at a local comp. Score of 39 from a BJCP judge
GT Keezer Build