This seems to be a fairly common question for which there is no good solution. There is simply no rule that allows you to sheathe a Katana for less than a Move action.
There are some alternatives however, although they do not fit strictly fit the brief of the question:
1. Gloves of Storing / Gloves of the Master Strategist
Both these items do the same thing, although the latter is much cheaper (3,600gp instead of 10,000gp) and allows the character to True Strike 1/day.
This device is a simple leather glove. On command, one item held in
the hand wearing the glove disappears. The item can weigh no more than
20 pounds and must be able to be held in one hand. While stored, the
item has negligible weight. With a snap of the fingers wearing the
glove, the item reappears. A glove can only store one item at a time.
Storing or retrieving the item is a free action. The item is held in
stasis and shrunk down so small within the palm of the glove that it
cannot be seen. Spell durations are not suppressed, but continue to
expire. If an effect is suppressed or dispelled, the stored item
This isn't sheathing the weapon, but you can make it appear and disappear easily, which may be sufficient: Can a Longbow be stored in a Glove of Holding?
2. The Gnomish Quickrazor
The gnome quickrazor is a wrist-mounded blade used in a quick,
circular slashing motion that snaps it out to strike, and then puts it
back into its rest in one smooth motion. This means that drawing and
stowing the quickrazor are both free actions, done as part of the
attack, though you are not considered armed with it in between your
attacks, during other creatures' actions.
This exotic weapon has the properties you need, but doesn't have the flavour. You could possibly just use the stats, but call it a Katana.
3. Have some help
A slightly tongue in cheek approach that I came across whilst searching for a solution:
- Take leadership.
- Have your cohort lie prone in your square.
- At the end of you turn, drop your katana.
- Have you cohort spend his whole turn picking up you katana and
putting it back in the sheath.
There's also a slightly less silly version using an Unseen Servant or a Spare Hand.
4. House Rule
If you really want to make it happen, just let players and NPCs with Quick Draw sheathe weapons for free. It probably won't break the game.
5. Sleight of Hand
Okay, this one is a stretch. If you are happy using a Variant Katana or a Paragon Katana (which might be homebrew?) or are willing to consider a normal Katana to be a 'small' object then you can effectively make it disappear using Sleight of Hand regardless of whether you pass your roll or not.
When you use this skill under close observation, your skill check is
opposed by the observer’s Spot check. The observer’s success doesn’t
prevent you from performing the action, just from doing it unnoticed.
You can hide a small object (including a light weapon or an easily
concealed ranged weapon, such as a dart, sling, or hand crossbow) on
... you may perform a Sleight of Hand check as a free action by taking
a -20 penalty on the check.
6. Additional Move Actions
Having an extra Move action to sheathe your weapon might be sufficient. The Belt of Battle (Magic Item Compendium), spells such as Lesser Celerity on a custom magic item and the Greaves of Aundair (Forge of War) are all potential ways to get a (limited) supply of extra Actions. These all require a Swift or Immediate action to be available however.