Check If Cast Is Possible

I use to import data from DBF Clipper databases into SQL Server. When a table is just imported its date fields have string format. I need to copy their data to tables of database where they ahve to be converted into date. Direct operator INSERT doesn't convert properly (I've not successed in changing default date format so it'll be covertable) but using CAST I can get result of strings like 13.05.1970 0:00:00 as datetime type. But not all records can be coverted this way. For ones can't be converted I've solved to make NULL fields there. But I don't know how to make CAST operation return NULL when convertion isn't possible. The query
INSERT INTO people_temp
(reg_num, surname, stname, patronymic, foreing, gender, birthdate, fam_pos, dwell_type, children, nation, par_not, region, stud_fml, parn_fml,
com_prob, sp_prob, sn_passport, nn_passport, dv_passport, wg_passport)
SELECT STUDENTs_temp.REG_NOM, STUDENTs_temp.FAMILY, STUDENTs_temp.NAME, STUDENTs_temp.PARN_NAME, STUDENTs_temp.INOSTR,

STUDENTs_temp.SEX,
CAST(PSPR_temp.DATA_BORN AS smalldatetime), PSPR_temp.SEM_POL, PSPR_temp.XAR_JT, PSPR_temp.CHILDREN,

PSPR_temp.NATION,
PSPR_temp.SV_ROD1 + PSPR_temp.SV_ROD2 AS Expr1, PSPR_temp.REGION, PSPR_temp.STUD_FML,

PSPR_temp.PARN_FML,
PSPR_temp.OB_STAJ, PSPR_temp.SP_STAJ, PSPR_temp.SN_PASPORT, PSPR_temp.NN_PASPORT, PSPR_temp.DV_PASPORT,
PSPR_temp.WG_PASPORT
FROM STUDENTs_temp INNER JOIN
PSPR_temp ON STUDENTs_temp.REG_NOM = PSPR_temp.REG_NOM
gets an error 'The conversion of char data type to smalldatetime data type resulted in an out-of-range smalldatetime value'. Tell me please how can make type casting return NULL if convertion isn't possible.