I found a solution: I changed UsersMembershipProvider password format to Hashed. Added new provider CustomUsersMembershipProvider with password format Encrypted. In this case I will validate password using new provider. Hope this solution will help somebody.

Create custom UmbracoOwinStartup class and change appStartup in web.config as <add key="owin:appStartup" value="Umbraco.Identity.OWIN.CustomUmbracoOwinStartup" /> Custom code:

  [assembly: OwinStartup("CustomUmbracoOwinStartup", typeof(CustomUmbracoOwinStartup))]
namespace Umbraco.Identity.OWIN
{    
    public class CustomUmbracoOwinStartup
    {
        public void Configuration(IAppBuilder app)
        {
            var applicationContext = ApplicationContext.Current;
            app.ConfigureUserManagerForUmbracoBackOffice<BackOfficeUserManager, BackOfficeIdentityUser>(
                applicationContext,
                (options, context) =>
                {
                    var membershipProvider = MembershipProviderExtensions.GetUsersMembershipProvider().AsUmbracoMembershipProvider();
                    var store = new BackOfficeUserStore(
                               applicationContext.Services.UserService,
                               applicationContext.Services.ExternalLoginService,
                               membershipProvider);
                    return new CustomBackOfficeUserManager(store, options, membershipProvider);                  
                });

            //Ensure owin is configured for Umbraco back office authentication
            app
                .UseUmbracoBackOfficeCookieAuthentication(ApplicationContext.Current)
                .UseUmbracoBackOfficeExternalCookieAuthentication(ApplicationContext.Current);         
        }        
    }
}
public class CustomBackOfficeUserManager : BackOfficeUserManager
    {
        public CustomBackOfficeUserManager(
            IUserStore<BackOfficeIdentityUser, int> store,
            IdentityFactoryOptions<BackOfficeUserManager> options,
            MembershipProviderBase membershipProvider) :
            base(store, options, membershipProvider)
        {
            // Call custom passowrd checker.
            base.BackOfficeUserPasswordChecker = new BackofficeMembershipProviderPasswordChecker();
        }      
    }
public class BackofficeMembershipProviderPasswordChecker : IBackOfficeUserPasswordChecker
    {
        /// <summary>
        /// Determines if a username and password are valid using the BackofficeMembershipProvider.
        /// </summary>
        /// <param name="user">User to test.</param>
        /// <param name="password">Password to test.</param>
        /// <returns>Object showing if user credentials are valid or not.</returns>
        public Task<BackOfficeUserPasswordCheckerResult> CheckPasswordAsync(BackOfficeIdentityUser user, string password)
        {
            // Access provider.
            if (Membership.Providers["CustomUsersMembershipProvider"] == null)
            {
                throw new InvalidOperationException("Provider 'CustomUsersMembershipProvider' is not defined.");
            }
            var adProvider = Membership.Providers["CustomUsersMembershipProvider"];


            // Check the user's password.
            var validUser = adProvider.ValidateUser(user.UserName, password) ? Task.FromResult(BackOfficeUserPasswordCheckerResult.ValidCredentials) : Task.FromResult(BackOfficeUserPasswordCheckerResult.InvalidCredentials);

            return validUser;
        }
    }