The poem describes a situation when three witches (Old Goody Jones, Goody Price, and Madge Gray) and a man (Robin) stole into the vicarage's cellar and indulged in revelry. These thieves laughed so loud that all the members of the vicarage awake and head to the cellar.
The next part goes...
On, on to the cellar! away! away!
On, on, to the cellar without more delay!
The whole posse rush onwards in battle array.
Conceive the dismay of the party so gay,
Old Goody Jones, Goody Price, and Madge Gray,
When the door bursting wide, they descried the allied
Troops, prepared for the onslaught, roll in like a tide,
And the spits, and the tongs, and the pokers beside! —
Can I change the bolded part into 'while Old Goody Jones, Goody Price, and Madge Gray were conceiving the dismay of the party (the whole posse) so gaily'?
I leave a link to the whole text here. https://ebooks.adelaide.edu.au/i/ingoldsby/thomas/ingoldsby_legends/chapter9.html