Want to create DAC extension for the existing field in acumatica table

Sure but what is the benefit of redundant field? Also, You can not have duplicate names in the same table as OrderType already exists in the POLine table. You will need to read/watch the basics of customizations including naming conventions before starting the customization. All custom fields should start with Usr otherwise on updates Acumatica will DROP those fields which are not an Acumatica standard field and do not start with Usr.

This is a sample of adding a custom field to PMProject (Contract Table)

using PX.Data;

namespace PX.Objects.PM
{
    // PMProject DAC Extension
    [PXNonInstantiatedExtension]
    public sealed class USRPMProjectExt : PXCacheExtension<PX.Objects.PM.PMProject>
    {
        public static bool IsActive() => true;

        #region UsrField
        public abstract class usrField : PX.Data.BQL.BqlInt.Field<usrField> { }
        protected int? _UsrField;
        [ActiveProjectTask(
            typeof(PMProject.contractID),
            CheckMandatoryCondition = typeof(Where<PMProject.isActive.FromCurrent.NoDefault.IsEqual<True>>),
            DisplayName = "My Field")]
        public int? UsrField
        {
            get
            {
                return this._UsrField;
            }
            set
            {
                this._UsrField = value;
            }
        }
        #endregion
    }
}