How to store particular columns of CDT in local Variable

There are a few ways to do this, might depend on your use case (?) and Appian version for which method to use.  First, you could take the fields you want and generate a local variable using a!map() (change to regular dictionary {} if not on a late version of Appian):

a!localVariables(
  /*local!abc: 'type!{urn:com:appian:types}Monitoring_Roster'(),*/
  local!abc: {
    field1: "a",
    field2: "b",
    field3: "c",
    field4: "d",
    field5: "e"
  },
  local!xyx: a!map(
    field1: local!abc.field1,
    field3: local!abc.field3,
    field5: local!abc.field5
  ),
  {
    local!xyx
  }
)

Or, utilize a local variable of the same type and only populate certain fields:

a!localVariables(
  local!abc: 'type!{urn:com:appian:types}Monitoring_Roster'(/* populated data */),
  local!xyx: 'type!{urn:com:appian:types}Monitoring_Roster'(
    field1: local!abc.field1,
    field3: local!abc.field3,
    field5: local!abc.field5
  ),
  {
    local!xyx
  }
)

Please provide some additional details on your use case for more specific answers.