No, you cannot.
And the reason for that, is because there are consequences of failure.
Retry? Yes, but after an initial failure, a second Sleight of Hand attempt against the same target (or while you are being watched by the same observer who noticed your previous attempt) increases the DC for the task by 10.
If you fail a Sleight of Hand check, you are discovered. So you cannot possibly know all people that will look at you or will search your body for that hidden knife. That means you will try to hide the item on your body at least twenty times, rolling from 1 (terribly hidden) to 20 (masterfully hidden). But how can you know how well you hid it without a feedback?
Since taking 20 assumes that your character will fail many times before succeeding, your character would automatically incur any penalties for failure before he or she could complete the task (hence why it is generally not allowed with skills that carry such penalties). Common “take 20” skills include Disable Device (when used to open locks), Escape Artist, and Perception (when attempting to find traps.
Taking 10, on the other hand, means you did try to hide the item using your best knowledge and made sure that it will be well hidden against almost anyone who tries to search for it, assuming you are skilled enough and trust your abilities (has ranks on it). But that does not count Luck, represented in the game system by the dice roll. So, on average, your dagger is very well hidden by taking 10.
In other words, Taking 20 assumes you will fail a few times.
Sleight of Hand is not a single check
Another point to take into consideration is that fact that you do not make a single Sleight of Hand check and oppose whoever attempts to find your concealed dagger, but one check per attempt.
If we look at Disguise, we have the following text:
You get only one Disguise check per use of the skill, even if several people are making Perception checks against it.
On Linguistics we also have something similar aswell:
The Linguistics check is made secretly, so that you’re not sure how good your forgery is. As with Disguise, you don’t make a check until someone examines the work.
Both skills mention that you only make a single check.
Sleight of Hand lacks anything like that on the skill description, it simply mentions it is an opposed check. Notice how Stealth also does not mention anything like that, so everytime someone rolls a Perception check to notice someone hidden, the characters must both check against each other (an opposed check), though they could say they are Taking-10 depending on the situation.