You can use this extension(FlashGot as core) to make direct calls to an external downloader (such as thunder, FlashGet and so on.)
Current supported DMs:
BitComet
Download Master
FlashGet 2
FlashGet 3.x
FlashGet Mini
FlashGet
Free Download Manager
FreshDownload
GetRight
GigaGet
HiDownload
InstantGet
Internet Download Accelerator
Internet Download Manager
LeechGet 2002
LeechGet
Mass Downloader
Net Transport 2
Net Transport
NetAnts
Orbit
QQDownload
ReGet
Star Downloader
Thunder (Old)
Thunder Mini
Thunder Web
Thunder
TrueDownloader
WellGet
iGetter
wxDownload Fast