There are a few phone wallets that allow you to convert private keys to seed phrases, but most do not.
You can't turn a Bitcoin private key into a seed phrase. A private key unlocks and allows you to spend coins associated with one particular address. A seed phrase is higher up in that pyramid structure and is the master key that gives you access to the coins in all addresses derived from that seed phrase. You can't use a key from a level below to magically turn it into higher-level master key.
...and if course I believe electrum wallet also offer the same services and feature.
Electrum wallet allows you to import individual private keys, but there is no "private key into seed conversion feature".
Nonetheless, keeping your seed phrase and private key apart is highly advised
There is no need to keep note and track of individual private keys in today's age. A seed phrase can derive all the private keys you would unnecessarily write down on paper.