Macrobius gives the form of a spell (an evocatio) designed to call forth the gods of an enemy city before attacking it, inviting them to Rome:
si deus, si dea est, cui populus civitasque Carthaginiensis
est in tutela, teque maxime, ille qui urbis
huius populique tutelam recepisti, precor venerorque
veniamquea vobis peto ut vos populum civitatemque
Carthaginiensem deseratis, loca templa
sacra urbemque eorum relinquatis, absque his abeatis ...
... propitiique Romam ad
me meosque veniatis, nostraque vobis loca templa
sacra urbs acceptior probatiorque sit ...
I call upon the one in whose protection are the people and community
of Carthage, whether it be a god or a goddess, and upon you above all,
who have undertaken to protect this city and people, and ask you all
for your favour: may you all desert the people and community of
Carthage, leave their sacred places, temples, and city, and depart
from them ... and come to Rome, to me and my people, and may our
sacred places, temples, city be more acceptable and approved ...
Saturnalia, 3.9.7 ff (trans. Robert A. Kaster)
Macrobius earlier clarifies the reasoning behind this tactic - the Romans believed either that a city could not be taken without the local gods first departing, or that it was against divine law to take a city in which its gods still resided (the existence and function of said gods being a given either way). To this end, he continues, the Romans tried to keep the Latin name of their city and its tutelary god a secret from their enemies (see 3.9.2-3). As a side note, this then is less a case of 'conversion' (or interpretatio romana), and more a sincere piety that extended to any and all gods encountered.