[juss]

as for convertion found 2 solutions:
from here https://forums.alliedmods.net/showthread.php?t=149587

First one

/**
 * Gives the 64-bit int SteamID for a client
 *
 * @param client        Client index of the player.
 * @param buffer        Destination string buffer.
 * @param maxlength     Maximum length of output string buffer.
 * @return              True on success, false on invalid client index or failure to find edict index.
 */
native bool:GetClientCSteamID(client, String:buffer[], maxlength);

/**
 * Given a 64-bit int SteamID, gives the SteamID associated with it
 *
 * @param input         64-bit integer stored in a string
 * @param buffer        Destination string buffer for the SteamID.
 * @param maxlength     Maximum length of the output string buffer.
 * @return              True on success, false on invalid input string.
 */
native bool:CSteamIDToSteamID(String:input[], String:buffer[], maxlength);

/**
 * Given a SteamID, gives the 64-bit int SteamID associated with it
 *
 * @param input         SteamID stored in a string
 * @param buffer        Destination string buffer for the 64-bit int SteamID.
 * @param maxlength     Maximum length of the output string buffer.
 * @return              True on success, false on invalid SteamID.
 */
native bool:SteamIDToCSteamID(String:input[], String:buffer[], maxlength);

and the Second one
Steam_GetCSteamIDForClient / Steam_GetCSteamIDFromRenderedID
from https://forums.alliedmods.net/showthread.php?t=129763


Or maybe is there other way?