I want to make a copy of a staff of healing and tweak it. I want it to be listed as an arcane focus among other things....

If I copy the staff of healing, I do not see a way to list it as an arcane focus though.

If I do a search for a basic "staff", it comes up and is listed as "Type:  Arcane Focus" though.

But I don't see a way to copy it.

Am I missing something?

There is nothing special mechanically about an “arcane focus” in the system, simply state it in the description and add the [FOCUS] tag to your item and bango, it’s a “spellcasting focus.” Actually, by default any “staff” is an arcane focus because the rules for an arcane focus say so. The staff of healing is not because it requires attunement be a Bard, Cleric, or Druid, so a Wizard cannot attune to it. I guess a staff that requires attunement must be attuned to use even that basic function. 🤷‍♂️

Do you want it be an “arcane focus,” or “spellcasting focus,” as they are not necessarily the same things.

• Clerics don’t cast “arcane magic,” but rather “divine” magi. So a Cleric can’t use an “arcane focus” but instead uses a holy symbol as a “spellcasting focus.” A cleric cannot use a staff as a holy symbol unless it specifically says so because they are not included under the rules for holy symbols. (Heck, if you have an amulet and some string or a reliquary on a piece of chain, a staff could be “converted.” 😂)
• Druids don’t cast arcane magic either, instead they cast “primal/nature magic.” A Druid an only technically use a wooden staff as a “Druidic focus,” not any old staff. One presumes a “staff” could be made of crystal or bone or metal or whatever, but the only one of those I see not working for Druids is metal, I dunno. (Alternatively they might be excluded from using a staff made of wood, but shot and reinforced with splints of steel while Wizards can use those.) But I once had to prove that a Wizard could use a wooden staff as an Arcane Focus just to prove that a Druid could use it as a quarterstaff without penalty. But a staff of healing doesn’t state it’s possible use as a Druidic focus, so I guess it’s not “wooden enough” or something. (However, if you’vegot a sprig of mistletoe or a totem and some string….)
• Bards at least used to be “arcane casters,” but that might have changed for this edition and I honestly do not know. Bards Technically, RAW a Bard can only use an instrument as a focus, nothing else. (If you’ve got a tantan and some string though…. 😉)

The reason they excluded traditional Arcane casters (Sorcerers, Wizards, Warlocks) from the list for the staff of healing’s holiday card list is because healing is the one thing they don’t do, and WotC wanted to maintain that division. But you do you, ain’t nobody gonna stop you.