I've converted several D&D spells to HARP using the CoM rules.
The key thing to know is that the process is not so much a "conversion" as a "rewrite inspired by the original spell": examine the original spell, determine what it does and how it does it. Make note of its attributes and restrictions (like duration, range, level, and so on). Then compare it to existing spells in HARP. Then, maybe, rewrite it in terms that are relevant to HARP (e.g. 3d6 damage for a fireball is a significant amount of damage in D&D, but mostly irrelevant in HARP - it should be a Fire critical).
Why "maybe"?
You'll find that for most D&D spells, there is already an existing HARP spell that does the same thing, or which has a very similar mechanical effect with perhaps a different flavour. The HARP core rules have over 100 spells, CoM adds over 100 more and around 35 cantrips. If you have the old Codex or Harper's Bazaar supplements, which are no longer available, they add about 150 more of varying quality.
Having lots of spells with similar effects (or the same effects scaled up for a higher-level spell) doesn't make as much sense in HARP as it does in D&D because:
1. HARP spells already have scaling options so they tend to improve as characters gain levels and increase their skill ranks in that spell, and
2. it's far less appealing to a HARP PC to have multiple similar spells with only a few ranks in each - they're generally better off having maximum ranks in just one of those spells and/or a wider variety of very different spells.
You may end up deciding that it's not worth the bother of converting that spell. It may make more sense as an additional scaling option to an existing similar spell, or just as descriptive flavour text for a similar spell.
If you decide that it is worth converting the spell then write a spell that does something similar using the HARP & CoM rules, and taking inspiration from other HARP spells. HARP spells should start off being relatively weak but have scaling options (and/or attributes like duration that increase with the caster's skill ranks, e.g. a duration of 1 minute/rank) that make them more powerful as the caster gains more skill ranks in that spell.
